Location sensing and tracking of moving objects has created a growing interest in numerous novel location based services and applications in various market segments. In the retail industry, for example, shopping carts equipped with personal shopping assistants enriched with some additional navigation functionality can guide customers through the store, provide them with location-based product information, and alert them to promotions and discounts as they walk through the aisles. However, customers will be satisfied only if this advanced shopping service can offer a tracking system which will accurately sense the location of the shopping cart. Determining at least the aisle in which the cart is located is crucial for obtaining customer satisfaction. The stringent requirement for estimating the location of the cart with an accuracy of less than one meter is a challenging situation. In addition, to make this system commercially viable it is essential that the total cost should be kept low.
Location tracking of objects in an indoor environment can be performed with various techniques, which can be based on mechanical, acoustical, ultra-sonic, optical, infrared, inertial, or radio-signal measurements. Among these systems, radio-based location positioning systems are most frequently used to sense and track the position of moving objects in an indoor environment. A radio receiver attached to the objects either measures the signal strength, the angle of arrival, or the arrival-time difference of received radio signals that are transmitted of multiple pre-installed reference transponder units. Since the locations of the radio transponders are known, a triangulation or signature method can be applied to determine the physical location of the moving object. The location estimates obtained with a radio-based location-positioning system are long-term stable, but are rather infrequently updated and suffer under a large error variance due the fading radio channel. Location estimates from measurements obtained with an Inertial Navigation System (INS), which comprises accelerometers, gyroscopes, and compass, are frequently applied in an outdoor environment (often in conjunction with the global positioning system) to track maneuverable vehicles like land crafts, aircrafts, and ships. INS systems can also be used for location tracking of objects in an indoor environment. These estimates are rather reliable, well-suited for short-term tracking, even though they suffer from drifts due to the integration operation required for the derivation of the location position from the acceleration and angular rate.
The present invention overcomes these problems and presents a solution for accurately estimating the position of a moving object in an indoor environment.
The following patents and papers are of particular interest for the disclosed tracking scheme:
The document U.S. Pat. No. 6,205,401 titled “Navigation system for a vehicle especially a land craft” describes a navigation system for a land vehicle, comprising one gyroscope, two accelerometers, and a velocity measurement device. In addition, a GPS receiver and/or map can provide position reference data. A Kalman-filter based approach is used to determine from the inertial measurements and the reference data the vehicle position and the direction of travel. The drawback of this invention is that it cannot be applied in an indoor environment because the reception of satellite signals within buildings is unreliable or even impossible due to wall shielding effects. Another document WO 2004/017569A3, titled “Transponder subsystem for supporting location awareness in wireless networks”, discusses an apparatus and method for determining the location of a communication device within an IEEE 802.11 standard based WLAN. The apparatus comprises transponders for communicating with the device when it is situated in the coverage area of the WLAN, and a processing unit for deriving the location of the device in dependence on information received from the transponders. For the determination of the location, measurement parameters such as received signal strength and/or time delay from a radio signal exchange between the device and transponders can be used. Adding inertial sensors to the positioning apparatus, and combining the radio-signal and inertial measurements with a Kalman-filter approach as proposed in the present invention can significantly enhance the positioning accuracy and reliability of the proposed method. The paper titled “Estimating optimal tracking filter performance for maneuvering targets” published in IEEE Transactions on Aerospace and Electronic Systems, vol 6, no 4, July 1970, deals with optimal tracking of manned maneuverable vehicles such as aircrafts, ships, and submarines. A Kalman-filter has been derived using a simple process model that closely represents the motions of the maneuvering targets in the 3-dimensional space. This paper describes the method of deriving from a continuous-time state-space description the equivalent discrete-time equations. However, the paper does not consider mode switching or forward-backward smoothing to enhance the accuracy of the vehicle tracker.